The various embodiments described herein generally relate to elevator installations. More particularly, the various embodiments described herein relate to detecting an abnormal travel behavior of a movable body travelling along a guide rail, such as an actual or potential derailment of the movable body.
Multi-story buildings are usually equipped with at least one elevator installation. In a generally known elevator installation, a suspension medium—such as a rope or flat belt-type rope—interconnects a counterweight and a cabin, and an electrical drive motor causes the suspension medium to move in order to thereby move the counterweight and the cabin up and down along a shaft or hoistway. In such an elevator installation, counterweight guide rails are installed in the shaft to guide the counterweight when it moves up and down. Similarly, installed cabin guide rails guide the cabin on its way up and down the shaft. These guide rails ensure that the guided bodies, i.e., the counterweight and cabin, stay within their defined spaces and paths, and, hence, do not collide with each other or with a shaft wall.
An earthquake poses a severe risk for an elevator installation in a multi-story building and the passengers using the elevator installation at the time the earthquake occurs. For that reason, certain elevator codes (e.g., European Code EN81 or US Code A 17.1-2010) require a safety mechanism that detects if a counterweight leaves a guide rail (also referred to as “the counterweight derails”) during an earthquake, or has left the guide rail as a result of the earthquake. A common safety mechanism is based on a ring-and-string concept: an energized cord (string) runs next to the counterweight up the length of the hoistway and passes through a ring attached to the counterweight. If the counterweight leaves a guide rail, the ring contacts the string. The contact causes the cord to be grounded via the counterweight, and a controller of the elevator installation reacts upon detecting such grounding. The clearance between the cord and the ring is relatively low, e.g., about 25 mm. Wind, however, may cause tall buildings to sway more than the clearance and cause the cord to contact the ring leading to false detections.
Patent abstract JP11035245 discloses a mechanism that detects a derailment when a target plate on a counterweight interrupts a laser beam. In tall buildings, the laser beam bridges the long distance between the bottom and the head of the shaft. Sway caused by wind may, therefore, also lead to false detections. Furthermore, positioning a laser source in a shaft may lead to other concerns, such as the safety of a service technician or optical degradation due to dirt on the laser source or the laser beam detector. Also, smoke caused by a smoldering fire or actual fire may interfere with the laser beam.
Another mechanism, disclosed in patent abstract JP07149482, uses a derail detector installed at a guide shoe. The derail detector detects if a slider contacts a guide rail. In order for such a mechanical slider to reliably function, the slider must be regularly serviced to ensure its mobility or to detect wear. Also, such a mechanical slider may cause additional noise, in particular at higher speeds.
In view of these known mechanisms and associated concerns, there is a need for an alternative mechanism for detecting an abnormal travel behavior of a movable body travelling along a guide rail.